Fluid flow control devices include devices for both liquids and gases. Valve actuators for fluid flow control devices are known and may be mechanically operated. For example, the valve actuator may be manually driven, operated by fluid pressure in which the shaft is connected directly or indirectly to a fluid operated piston, or be driven by an electro-hydraulic or electro-fluid means. Conventional valve actuators comprise an electrically driven input shaft, which may be rotatable at relatively high speeds with relatively low torque. The input shaft may, through reducing gears such as a worm gear or a helical screw thread and nut, rotate a relatively high torque, low speed output shaft.
It may be desirable to determine the torque generated by the output shaft. For example, when a valve is fully closed and seated, the torque required to open the valve may be considerably higher. Consistently monitoring the torque may indicate if a valve is wearing out or sticking. Trending patterns in the torque measurements may enable predictive maintenance. Override shut-off features may be provided if a torque exceeds a predetermined allowable level.
Measurement of the axial force on the input shaft may be used to determine the torque delivered by the output shaft. The axial load multiplied by the worm gear pitch radius is the torque delivered by the output shaft.
Conventional devices for measuring the end thrust or torque of a rotating shaft are known and include a thrust-torque transducer described in U.S. Pat. No. 4,182,168 to Desch. The thrust-torque transducer includes a LVDT (Linear Voltage Differential Transformer) having a movable core axially aligned with, secured to, and rotatable with the shaft, and producing an output signal corresponding to thrust or torque. However, in order to provide for operation of the transducer in both clockwise and counterclockwise rotations of the shaft, the Desch thrust-torque transducer requires presetting of a diaphragm of a thrust bearing. The Desch thrust-torque transducer does not detect any misalignment of the axial load on the shaft.
Another conventional device for indicating loading on a shaft is described in U.S. Pat. No. 5,503,045 to Riester. An increased load on a worm causes axial shifting of a worm shaft and an accompanying deformation of a membranous disc mounted on the worm shaft. One side of the disc is formed with a circumferentially extending, annular recess. The central portion of the disc is fixed against axial displacement relative to the worm shaft by an axial bearing situated on one side of the disc and a bushing which is disposed on the opposite side of the disc. A strain measuring strip on another side of the disc generates changes in measurements with displacement of the worm shaft. The device of Riester does not provide a method for detection of any misalignment of the load on the shaft.
Therefore, it would be advantageous to develop a technique for measuring the torque generated by an output shaft using the axial displacement of an input shaft, and detecting any misalignment of the load on the input shaft.